Electrical equipment requiring low voltage DC are frequently energized by mains operated power supplies. FIG. 1 shows schematically a conventional startup circuit 1 in a typical low voltage power supply, wherein mains voltage 2 is rectified typically by a bridge rectifier 3 and then fed via a resistor 4 to a control circuit (not shown) in the power supply. The input voltage to the control circuit is maintained at a required level by a zener diode 5 connected in parallel with a capacitor 6.
During operation, the resistor 4, which will be referred throughout as a “starting resistor”, feeds current to the capacitor 6 which therefore charges to a value determined by the zener diode 5, thus ensuring a constant voltage input to the control circuit Typically, the mains voltage is 110 VAC in the USA or 220 VAC in Europe, while the equipment operates on a much lower voltage, such as 30 volts or even less. The startup circuit 1 serves to energize the power supply directly from the mains supply after it is first switched on in a controlled manner. However, once the power supply is operating and has reached a steady state voltage, there is no longer any need to supply energy to the starting circuit, which is now redundant.
A drawback with the arrangement shown in FIG. 1 is that even under steady state conditions, when the starting circuit is no longer necessary, the constant flow of current through the starting resistor 4 manifests itself as a constant energy loss, thus reducing the overall efficiency of the power supply. The amount of power dissipated in the starting resistor 4 is a function of the difference between the input voltage and the output voltage, since the closer the output voltage is to the input voltage in the steady state, the less is the voltage dropped across the starting resistor and therefore the lower is the energy loss therethrough. In power supplies designed to operate from a single voltage power supply only, it is possible to optimize the circuit components so as to reduce the constant energy loss through the starting resistor. However, in so-called universal power supplies that are intended to operate over a range of power supply voltages, such as 85-277 VAC so as to be suitable for both the US and European markets, such optimization is difficult to achieve and it becomes impossible to minimize the energy loss through the starting resistor for all supply voltages.
It would therefore be desirable to dispense with the starting resistor once the power supply is operating normally and reaches steady state. The prior art has recognized this need although apparently not in a universal power supply. Thus, reference is made to FIG. 2 showing a prior art power supply 10 disclosed by JP 2001275347 published Oct. 5, 2001 and assigned to Toshiba Lighting & Technology Corporation. The reference numerals shown in the figure are those that are appear in the abstract of this publication, and only the relevant components will now be described.
The power supply unit 10 includes a starting resistor 17 that feeds the output from a bridge rectifier 13 to a control circuit 16 on startup via a first transistor 18. A startup circuit feeds the output from the bridge rectifier 13 to the first transistor 18, thus maintaining the flirt transistor 18 conducting during starting and feeding power to the control circuit. A second transistor 21 is driven by a potential difference between the input and the output of a voltage regulator 22 and maintains constant voltage generated in a primary auxiliary winding 15b of an output transformer 15 after startup. The second transistor 21 feeds the resulting voltage to the control circuit, which is driven thereby, and inverts the first transistor from conduction to cutoff thereby effectively disconnecting the starting resistor 17.
Thus, the power supply unit saves electricity during standby by separating starting resistance after a startup (of a switching circuit), and driving the control circuit of a main switching element by only power generated in an output transformer.
It will be seen from FIG. 2 that an electrolytic capacitor 23 is connected across the input immediately after the bridge rectifier 13. The purpose of the electrolytic capacitor 23 is to store energy from the mains and serve as an auxiliary supply in the event of a momentary outage or fluctuations in the main voltage. In order to serve this function, the capacitor 23 must have a high capacitance and indeed this is the reason that an electrolytic capacitor is employed. However, the connection of a high capacitance at the input of the circuit militates against the power supply having near unity power factor. This may not matter too much when the power supply is to be used with computers and the like. However, there are many applications where near unity power factor is required and, in such cases, the circuit shown in JP 2001275347 is unsuitable.
In order to achieve near unity power factor, a high capacitance of the order of 200 nF is usually disposed near the output of the power supply. This increases the time that it takes for steady state to be reached and this in turn increases the time before the startup circuit must be disabled. In JP 2001275347 the time taken between the first switch 18 opening and the second switch 14 closing is too fast to allow complete charging of such capacitance. This also indicates that the circuit disclosed in JP 2001275347 is unsuited for use with power supplies having near unity power factor.
It would therefore be desirable to provide a startup circuit for a power supply, particularly a universal power supply having near unity power factor, wherein the starting resistor is disconnected after the power supply has reached steady state, thereby preventing energy loss and improving efficiency.